Aerated lagoons or ponds are one way of treating municipal wastewater. These bodies of water are mechanically aerated to oxygenate and to mix water. In northern climate conditions such as those found in Canada, these lagoons typically comprise 2 to 4 basins offering large volumes with corresponding minimum retention time typically between 15 and 60 days to achieve less than 25 mg/l 5-day carbonaceous biochemical oxygen demand (CBOD5). The 5-day carbonaceous biochemical oxygen demand (CBOD5) represents a measure of the rate of oxygen uptake by micro-organisms at 20° C. in 1 litre of wastewater over 5 days and is a typical wastewater quality indicator set by legislative authorities in some countries. Other quality indicators include Total Suspended Solids (TSS), typically measured through sieves varying between 0.45 and 2.0 μm after a sample is evaporated to dryness, Total Kjeldahl Nitrogen (TKN) representing the sum of organic nitrogen, ammonia (NH3) and ammonium (NH4+), and Total nitrogen (TN) which includes TKN as well as nitrates (NO3−) and nitrites (NO2−).
Although lagoons do not have a high treatment efficiency compared to tank-based water treatment systems, they are the most popular municipal wastewater treatment in Canada with more than 2000 lagoons. In the province of Québec alone, there were 633 lagoons in 2013 amongst 811 municipal treatments. Land availability, availability of skilled operators, high dilution capacity of receiving water bodies and non-restrictive discharge requirements and economic aspects have led to the popularity of lagoons. However, urban development and environmental pressure requiring more stringent discharge requirements are challenging the past advantages.
In an aerated lagoon with no sludge recirculation, biomass is found in suspension in concentrations in the order of about 100 to 400 mg TSS/l. A clarifier near the outlet can increase biomass concentration, but the operation of clarifier equipment requires skilled labour, expensive equipment, and is not possible in cold climates especially when there is an ice cover.
Furthermore, due to the low kinetics of nitrifying bacteria especially at temperatures below 8° C., the poor hydraulic efficiency of lagoons and the low biomass concentration, nitrification in some countries is possible only during the warmer months, for example during part of the summer under the 47th parallel in Canada. Nitrogen removal through denitrification is seldom observed in lagoons.
Therefore, there is a need for water treatment systems and methods which overcome or reduce at least some of the above-described problems.